When Worlds Collide
by SafetyNerd
Summary: It has been a couple years since the monsters reached the surface. Inter-species relations are decent, and everyone is happy. That is, until one day, when an unfamiliar door appears in the Dreemurr home. A gateway bridging two worlds that never should have touched. What happens when Frisk decides to enter the door?
1. Prologue

"Interesting..."

A voice spoke out in the darkness.

"Very interesting..."

As the voice spoke, it felt as if the darkness was getting darker. Somehow, pitch black was darkening. Just before he descended into complete madness, Sans woke up.

"That's gotta be the fifth time in a row I've had that same dream."


	2. The Door - Chapter 1

Frisk sat up in bed and rubbed their eyes. Today marked the two-year anniversary of the day that monsters gazed at a beautiful sunset. For some, it was the first time they had seen it in years. For others, it was the first time at all.

Or was that tomorrow?

Nonetheless, Frisk changed into clothes more suitable for the daytime and left the room. The inviting scent of hot maple syrup filled the air. It wasn't that unusual, as having a mother like Toriel meant eating a complete breakfast every morning.

"Good morning, Frisk," said Toriel, in her normal, gentle manner. The table had already been set. The selection of food included waffles, scrambled eggs, and bacon. A nice, generic breakfast. "We had better eat quickly. The final preparations for the festival are being made today." So it WAS tomorrow. Toriel continued, "I would like you to help us as much as possible."

The festival had been planned for about a month by the mayor of East Ebott Town (Frisk's current place of residence). Papyrus enthusiastically agreed to help, despite never being asked to. He ended up supplying roughly half of the ideas used for the festivities.

After eating, the two prepared to leave. Setting up the remaining aspects of the celebration would likely span across the entire day. They walked out into the cold air. It was only March, so there was still snow on the ground. Two minutes later, with the brilliant reaction time of a snail, Frisk realized they had forgotten their coat. They told Toriel to keep walking, and that they would catch up to her. "I hope this isn't a trick to get out of working," Toriel said. Frisk shook their head frantically. "Very well, then. I will see you in a few minutes, my child."

Frisk trudged through the snow back to the house and walked upstairs to their room. Before entering, however, they noticed another door on the far end of the hallway. Oddly enough, they had never been in the room or even noticed it was there. It was almost as if the door was emitting... darkness.

Frisk reached for the doorknob and slowly turned it. The room looked like a really dark closet. Reluctantly, they stepped inside.

\--

In another part of East Ebott Town, a pair of skeletons were messing about. "Sans!" Papyrus shouted from the living room, while putting on his coat. Sans poked his head out of his bedroom to see what was up: "Yeah, bro?" "We have been here for nearly two years and you still haven't picked up your sock!" "Isn't that what you told me to do yesterday?" Papyrus would've rolled his eyes if he had them. "Yes, and also the day before, because it is THAT IMPORTANT!" "Eh," Sans shrugged, "I'll do it tomorrow." "That's what YOU said yesterday, and the day before! You need to fix your procrastination problem!" "Nah," Sans disagreed, "I'll also do that later." Papyrus groaned, indicating that he was done trying to take care of the sock for the day.

Papyrus finished buttoning his coat. He, of course, didn't need a coat, but he still wore it to "look cool." He looked at his brother again. "We are going to be late if you don't hurry up and get ready!" Sans shrugged again. "Go on without me, Paps. I have something I need to do first. Don't worry, I'll just use one of my shortcuts." "If you say so, Sans."

A short amount of time after Papyrus left the house, Sans walked to their "unused" shed and checked the display on one of the several machines in the room. "Just as I thought. The anomaly sensor is showing increasing readings. Today is far worse than the past week, though." Sans shrugged for the third time that morning. "Oh well. Not my problem." Wasting no more time, he left the shed and locked it, then teleported to the town hall.

\--

Frisk took a few more steps into the room, which more accurately fit the description of "a really long hallway." They had been walking for at least five minutes. Suddenly the door was shut. Frisk ran back towards it, but discovered that there was no longer a knob. The hallway got darker.

A feeling came over them that they recognized. Frisk was falling. They fell for a while, and then... *thud*


	3. Newcomer - Chapter 2

**Thank you, cefmlp for the follow and favorite!**

 **I'll do my best at updating this story regularly (just like any other writer wants to do). The reason that the final thing is formatted strangely is due to the mobile app's quirks. At least it's still readable** **(more or less).**

Susie and Kris walked through the forest. "Y'know..." Susie said, "Lancer did a really good job at improving this place. It feels like it was only yesterday that there were enemies trying to murder us" (and Susie trying to murder enemies).

Ralsei caught up to the duo, having fallen behind after patting a C. Round on the head for a solid four minutes. "That's because it WAS yesterday, Susie!" Ralsei pointed out. "Fair enough, but I felt like I waited for a long time to come back." Neither Kris nor Ralsei wanted to try to understand Susie's logic, so they just continued walking in silence.

\--

Meanwhile, at the cliffs, another human was waking up.

Frisk looked around at the unfamiliar surroundings. Dark? Check. Creepy? Check. Underground? Probably, but this wasn't the underground that they knew, despite falling not too far from Mt. Ebott. The area reminded them of a combination between the true lab and the ruins.

They stood up and started walking in the only direction they could. Unlike the places previously mentioned, it didn't seem like there were any living things here. That is, except for the dark figure that seemed to be following them. Frisk scanned the area again and discovered that the dark figure was gone. They continued walking. A voice behind them spoke out in the darkness.

"Lightner... Turn around and face me." Frisk turned around to see a monster that appeared to have a black spade (like the ones on playing cards) in place of his eyes. He was also constantly sticking his blue tongue out. "Isn't that better?" he asked, "Now we can see each other properly!" Frisk just stared blankly at him. "What? You act like you've never seen me before!" Frisk shook their head. "Oh... Well, I'm Lancer! I'm the king of this... uh... kingdom!"

Frisk, who already knew what to do in the presence of royalty, bowed down. Lancer looked surprised. He explained, "You don't have to bow to me. Just follow me!"

Lancer led Frisk to a door that was similar to the one that led to Sans's bedroom. "Go to 'Castle.' I'll be there soon!" As soon as Frisk entered the door, a menu popped up. They selected "Castle," then stepped outside. The gateway had transported them to the outskirts of a castle.

A few seconds later, Lancer appeared through the door. "This is my castle! It used to be my dad's, but he was naughty, so we put him in time-out!" Frisk gave a thumbs up and followed the king into the palace.

Not too long after Frisk and Lancer entered the castle, Kris and co. reached the building. Susie looked eager to go inside. "Hey Susie," Ralsei inquired, "Is it me, or do you look excited?" Susie, as per usual, got on the defensive: "N-No! It's just you!" Several seconds passed. "Okay, maybe I'm a LITTLE excited to see Lancer again. But you would be, too, if you were visiting one of your best friends!"

The group opened the door to find Lancer talking with Frisk. "Hello, everyone!" Lancer expressed, "I brought another Lightner!" Frisk waved at the squad. "Huh," uttered Susie.


	4. Time to Bake! - Chapter 3

**Thank you to Ultrahero74, Shanelle Anjelee Monegro1, QueenLunaCrystal, RemiBouchard, LM1991, Dark156, Andrea and Naturberd, Shadowtalon62, jc351612 for the follows and favorites** **! Also, thank you to UNNES1000 for the positive review!**

 **I apologise for the time between each chapter, especially with how short the chapters are. Life has just been busy with the holiday season. Anyway, I'll continue the story.**

Susie was the first to break the silence: "I was beginning to think that Kris was the only human." "Well," Lancer stated, "I found another one! They fell down just like you two did!" Ralsei greeted them: "It's nice to meet you..." He trailed off, as if waiting for something. Frisk told him their name and he finished his sentence. "Oh, Frisk! It's nice to meet you, Frisk! My name is Ralsei! This is Kris, and this is-" Susie cut him off. "Susie. I can introduce myself, Colgate."

"So..." Ralsei asked, "What do plan to do while you're here?" "You DID promise to bake a cake for us," Susie answered. "Oh yeah, that's right! To the kitchen! That Lancer probably has in his castle!" Lancer grinned and led the group to the castle's kitchen.

"Why can't we use your kitchen?" Susie questioned Ralsei. "My place is... Not working... Properly." "What is that supposed to mean?" "It doesn't matter... Let's make this cake!"

\--

Two hours later, Kris and Frisk were using fire extinguishers to put out Ralsei's cake. "Why would you put salsa in a chocolate cake?!" Susie asked angrily. Ralsei, appearing more uncomfortable than panicked, replied, "Lancer said it would taste good..." "Whatever..." "Why don't we ask the yellow chef guy to make us a cake?" Lancer suggested, "Besides, you used up all my ingredients in the cake." Kris mentioned that Top Chef DID bake delicious pastries.

Ralsei had a determined glint in his eye. "I promised to bake the cake, so I will bake it myself! I just won't put salsa in it this time! We can go to my c- er... we can go to a shop and buy more supplies!" "Why can't we just go to your place?" Susie inquired. Ralsei got even more uncomfortable with the mention of his castle. Kris looked at him with concern in their eyes. Even Frisk knew something was wrong.

"C'mon, guys. We're gonna go to wherever it is that Ralsei lives," Susie instructed. Ralsei couldn't prevent this from happening, seeing as he was outnumbered in opinion. The poor pile of fluff just frowned and stared at the floor as the team (plus Frisk) left for his home.

It was a quiet journey. Even Lancer said nothing the entire time.

Along the way, Frisk noticed a shiny, metal object on the ground. It looked like a broken piece of a key. They picked it up and put it in their inventory. Perhaps it would be important later on.

They passed a doorway that reminded Frisk of Gerson's shop. Inside was a monster that looked similar to a tattered cat doll. The room was lit up by a single, dying lantern. Frisk decided they would check it out some other time.

Eventually, they reached the still-deserted town. It seemed much darker, which was unusual due to the low amount of light present in the underworld. The group approached Ralsei's castle. "What is THAT?!"

 **Ah, nothing like short chapters with minor cliffhangers at the end. I have some planning and researching for the story to do, but I should be able to write the next chapter(s) soon.**


	5. Glitches and Darkness - Chapter 4

**Thank** **you to thomasponyfan1, XxxMBDDXXX A.T.F.R., muddyleaf, and cinnamon.jingoku for the follows and favorites!**

"What is THAT?!" Susie asked while pointing to Ralsei's castle. The structure was covered in odd, unstable polygons. "This is why I didn't want to come here..." said Ralsei. Lancer stared up at the glitchy building and disagreed, "I don't see anything wrong with it! Let's look inside!" "Wait, but I-" Ralsei tried to stop him, but everyone had already gone through the doors. He sighed.

\--

Back on the surface, it had been far more than "a few minutes." In fact, it had been three hours since Frisk went back to get their coat. This also meant that Toriel started worrying two hours and forty-five minutes ago.

"Frisk has been missing for over two hours," she told Papyrus, "Have you seen them?" "I'm afraid I haven't, Toriel. Where was the human at the time of this disappearance?" "They went to our house to retrieve their coat. Could you go there to check on them?" "I would do anything for a friend! I shall depart at this very moment!" Immediately after this remark, Papyrus set out for the Dreemurr household.

The skeleton ran as fast as he could. Upon arriving, he discovered that the door was still unlocked, which was good for Papyrus since he forgot to ask for the keys or bring his own. "Human?" he called out, "Are you in here?"

But nobody came.

\--

Frisk felt a shiver run down their spine as the group entered Ralsei's castle. It was too dark to see anything in the room. "Any chance you got a light switch in here?" Susie asked. Ralsei shook his head. "No, but there are usually lanterns on the walls. They haven't been working lately. Sometimes even the walls themselves have problems..."

Susie put her arm straight through a wall. "Wha-? It's not solid..." Lancer also put his arm through the wall and waved it around. "Dude, your place gives me the creeps. Let's just leave..." Susie said. Kris, who had been leaning next to the door for some time, pushed it open, and the gang left the castle.

As they walked through the castle town, Kris pointed out that the dark rectangles were spread out farther than before. Ralsei still appeared to be worried. Lancer, who remained oblivious to the situation, asked, "What's so bad about the weird shapes, anyway?" "It's not supposed to be there," Ralsei explained, "Like a... Like a glitch." Frisk recalled the grey door in waterfall. It would rarely ever appear when they walked past the area. There were also the "followers," creepy versions of monsters in the underground that didn't seem to actually exist. Maybe the two events were correlated?

\--

Papyrus stared at a door at the far end of the hallway. He felt like it was calling to him. " _Papyrus..._ " a voice whispered, " _Come closer..._ " As if in a hypnotic trance, he crept towards the door. " _Open it..._ " He reached for the doorknob.

\--

Sans walked around East Ebott while carrying an instrument in his hands. He liked to call it a "Spacey-Wacey Detector," just because it sounded eccentric. It was designed to scan for dimensional disturbances in reality, such as entrances to other universes.

The signal seemed to grow stronger as he got closer to Toriel's house. "What could possibly be there?" he wondered. He turned the doorknob. It was already unlocked. As he entered, the readings increased. He went into a hallway and looked at the far end.

"Papyrus?" Sans asked, "What're you doing here?" Papyrus snapped out of the trance and faced his brother. "I was just searching for Frisk. Are you doing that, too?" Sans hid the Spacey-Wacey Detector behind his back. "Uh... Yeah. I can take it from here. You should be helping out with the festival, anyway." "I suppose you are right... I will see you later!" "C'ya, bro." Papyrus headed back for the park.

Sans pulled out his Spacey-Wacey Detector again and pointed it towards the door at the far end of the hallway. He stepped up to the door and the readings started going haywire. "..."


	6. Update (1-21-19)

**I apologise for the lack of updates recently. Due to unforeseen circumstances (I never thought I'd say/type that), I won't be active on FanFiction for a while. Sorry about that, and I hope to return as soon as possible.**


	7. Missing in Action - Chapter 5

**I'm back, finally! It's been a while (nearly 6 months), but I think I have enough motivation to continue writing this story.** **I've also decided to start writing slightly longer chapters than before.**

Ralsei stopped. "Hold on," he said, "Has anyone seen Frisk or Susie?" Lancer glanced around and answered, "Nope! They wandered off without us!" Kris noted that Susie had been unusually quiet since they left the castle. Frisk wasn't as much of a problem, as they were always quiet.

"Well, there's Frisk," Ralsei pointed at Frisk, who was carrying a dish full of Spin Cakes. They had decided to look for Top Chef and get some cakes. On the way back, they were distracted by a side path. It led to a chest containing another key piece, which was the exact same color and design as the first.

"I hate to say this, but we need to go back to my castle," Ralsei said, "Susie might've gotten lost there." Kris gave a thumbs up. As the group went on their way, Frisk decided that they would check something else.

Frisk walked to the shop they saw earlier. They asked the shopkeeper, Seam, about the two key pieces. Seam frowned and said grimly, "After all the trouble I went through to hide the pieces..." Seam's frown became a forced grin. "That's odd... I'd always imagined that I would be asked about a 'strange prisoner' first. Oh well. Just take the other piece. It doesn't matter much, anyway..." Seam handed Frisk the other piece of the broken key.

\--

Ralsei, Kris, and Lancer arrived at the dark castle town. All of the houses were now covered in the glitchy substance.

"Susie?" Ralsei called out. He stopped to listen for a response, but nobody answered. "Oh dear... What do we do?" Kris suggested they go back to the surface. "That's not a bad idea. Let's go!" The group agreed, as the area was creeping them all out.

The following trip was also eerily silent. With the loss of two people and the danger of glitches, the tension was overwhelming. Even the C. Rounds weren't too happy.

Once they reached the gateway to the light world, Ralsei stopped them. "Kris, I don't know if Lancer and I can travel to your realm. What if the light kills us?" Kris assured him that they would be fine, and that the worst the sun can do is give them sunburns. "Okay..." Kris stepped into the glowing portal, followed by Lancer. Ralsei hesitantly joined them.

\--

Sans sighed. It was getting late and he still found nothing but ridiculous rift readings. Toriel entered the house, exhausted after a day of work. "Sans, what are you doing here? I thought you were helping with the festival." He looked at her. "Sorry, got distracted by your house. I think it's infested with something." Toriel, despite her fatigue, gasped, "Oh dear! Do you think it has something to do with Frisk's disappearance?" Now it was Sans's face that put on a look of worry. "Frisk is missing?" She nodded and elaborated, "Yes, they've been missing all day." If Sans had become even more stressed, he clearly didn't show it. He promised her, "Don't worry, Tori. I'll find them. In the meantime, go over to my house. You can sleep there while I solve this mystery." "Thank you, Sans." She left for the skelebros' home. "Welp, things just got a little more complicated," Sans said.

He began scanning the door again, when he heard a loud noise come from Frisk's room. It sounded like a thud, as if someone had fallen out of bed. He crept into the room and turned on the light.

"Frisk?" he asked, but the human was nowhere to be seen. Instead, he spotted a monster that looked like a purple dinosaur with dark brown hair. The monster looked up at him. "Do I LOOK like a Frisk to you?!" Sans apologized, "Sorry, thought you were someone else." "Whatever. Aren't you that skeleton that just moved in?" He shook his head and replied, "Nope. Been here for two years. Do I know you? Or are you just a burglar who's bad at their job?"

Susie looked around the room. "Nevermind that. Where am I?" "Uh... Frisk's room in Toriel's house in East Ebott Town in-" Susie cut him off, "That's enough, thanks." "Anyway," Sans inquired, "How'd you get here, exactly?" Susie explained the hole in Ralsei's castle, which led to her present location. "Huh, interesting," he said, "Welp, it's getting late. You can sleep in Frisk's room and I'll take the couch." Susie reentered the room and promptly fell asleep. Sans walked to the couch and decided to get some shut-eye as well.

\--

A similar situation arose with Kris's group. They convinced Toriel there to house Ralsei and Lancer, Ralsei in Asriel's old room and Lancer taking the bathtub in the bathroom (his own personal request). It was difficult to explain, but it worked out in the end.

\--

After receiving the final piece and getting the key fixed by Malius, Frisk headed to Card Castle to find the cell of this "strange prisoner." They wandered around for half an hour until they noticed an extra button on the elevator that read, "Floor ???" They pressed it and waited for the lift to reach its destination.

Once there, Frisk stepped out and walked down an unnecessarily long flight of stairs. There it was – the entrance to the cell. "Am I mistaken, or do I sense a visitor has arrived?" a voice asked from the other side of the door. The sound sent a chill down Frisk's spine. "Ah, yes. You must be the one I have been told about! You are the human who is meant to start what happens next!"

Frisk looked down at their arm. It was moving on its own, the key in hand. Despite their best efforts, there was no stopping it. The door was unlocked and opened. A jester-looking creature flew out. "Thank you, thank you! Now I can free those of both worlds from their prison!" Without another world, the jester vanished. Frisk looked down. They had just put their friends in danger. They threw the key into the cell in frustration, then took the elevator back up.

Wandering around, Frisk discovered a gateway that reminded them of the barrier. They stepped in, taking them to the surface of that universe. They walked through the town and eventually came upon the skelebros' house and knocked on the door. Sans answered, "Do I know you?"


End file.
